1. Field of the Invention
This disclosure generally relates to optoelectronic reading devices for acquiring images of machine-readable symbols, such as bar code, area code, matrix or stacked code symbols, and particularly to providing autofocus capability in such devices.
2. Description of the Related Art
Optoelectronic readers for reading bar codes and other machine-readable symbols fall into two general groups: 1) moving beam devices commonly referred to as scanners (e.g., laser scanners), and 2) fixed beam devices commonly referred to as imagers (e.g., CCD imagers). Each group has its own relative advantages.
Moving beam devices typically move or scan a light across a target. For example, a scanner may employ a laser diode and a mechanism for moving a laser beam across the target. While it may be possible to move the light source itself, scanners typically employ one or more rotating or oscillating mirrors which reflect the laser beam, sweeping back and forth across a target, thereby sequentially illuminating portions of the target along a scan line. Scanners also typically include an electro-optical sensor, such as one or more photodiodes, that detects the laser light reflected from the target and produces a corresponding analog signal. The scanner may employ a standard lens or retro-collector to focus the reflected light on the electro-optical sensor. Typically, the scanner, or an associated device, converts the analog signal to a digital signal before decoding the digital signal according to standard decoding schemes.
While fixed beam devices may rely on ambient light, most imagers employ an illumination system. The illumination system typically includes a number of high-intensity light-emitting diodes (LEDs) arranged to simultaneously flood the entire target with light. Imagers strive for uniform illumination over the entire target. Imagers also include an electro-optical sensor (e.g., one- or two-dimensional CCD arrays), and may include a lens system to focus reflected light onto the sensor. A CCD array may be electronically sampled or scanned, to produce a digital signal suitable for decoding. Imagers eliminate moving parts, and allow high reading speeds at relatively low cost.
Many machine-readable symbol readers employ a fixed focus lens. A typical design distributes approximately one thousand pixels of a sensor over a 100 mm field-of-view at ˜150 mm, achieving a maximum barcode resolution of 0.1 mm per bars (Nyquist limit). Yet in many applications, the reader must provide a relatively large tolerance with respect to the distance between the target and the lens. To increase tolerance, the ratio of the image distance to the aperture lens diameter for finite-conjugate situation or the ratio of the focal length to the aperture lens diameter for an object at infinity (“f-number”) is increased as much as possible. The increase is limited by the minimum illumination required by the sensor, and ultimately the pupil diffraction blurring.
To compensate for the consequent reduction in reflected light (i.e., in inverse proportion to the square of the f-number), barcode readers typically include a larger number of, and/or higher intensity, light sources than would otherwise be required. For example, some barcode readers employ red LEDs to achieve a higher intensity output per watt. Most readers with red illumination also employ a ‘black and white’ sensor without a mosaic filter on top of the photosensitive matrix of pixels, since green and blue pixels are blind to red illumination. With a Bayer filter (FIG. 4), the cut-off frequency for red radiation is two times lower than for a ‘white and black’ sensor, and the modulation transfer function is higher at the aliasing frequency (See Modulation Transfer Function in Optical and Electro-Optical Systems, D. Boreman, SPIE press).
Thus, while a high f-number allows large depth-of-field (i.e., with a lens focused at hyperfocal distance), as a practical matter the range of illumination restricts operation to short distances (e.g., below 1 meter) to maintain a sufficient signal-to-noise ratio. Additionally, while fixed focus barcode readers are cost effective, they are highly specialized devices, typically having a short range and limited to monochromatic uses, and thus incompatible with many other imaging applications.
While some machine-readable symbol readers provide an auto-focus functionality, such devices typically rely on changing the internal spacing of one or more of the elements of a lens system along the optical axis to achieve variation of the focal length. Moving lens systems disadvantageously require relatively large and precise mechanical lenses, tracks, linkages and drives. Moving lens systems are also rather slow in responding, making such systems impractical for most automatic “on-fly” barcode reading.
Other attempts have been made at providing auto-focus capability in barcode readers. For example, a designed proposed by U.S. Pat. No. 5,574,598 (Nippondenso) employs an elastic change of the curvature of a transparent fluid-filled lens. A design proposed by Vdovin, et. al. “Micromachined mirror with a variable focal distance in Free-Space Microoptical Systems”, Digest of EOS Topical Meeting, pp. 28-29, Apr. 1-3 1996 employs electrostatic deformation of the curvature of a mirror. A design proposed in U.S. Pat. No. 6,464,363 (Nishioka et. al.) employs an electrostatic mirror. Finally, a design proposed in U.S. Pat. No. 6,347,742 (Winarski et. al.) employs a variable focal length liquid crystal lens. Such compact adaptive lenses rely on the rotation of the liquid crystal under an electric field that produces a change of refraction index. Liquid crystals have slow response times and exhibit polarization dependence. Additionally, the thickness of the crystal film reduces the optical transmission, limiting the auto-focus range. The transmission is further reduced if unpolarized light is or when two orthogonally aligned liquid crystal lenses are used. Principles of liquid crystal lenses are further described in “Adaptive Optical Components Using Liquid Crystal Devices”, G. D. Love, Journal of the Communications Research Laboratory, Vol. 46 No. 3, November 1999, pp. 427-430.
There is a need for a reliable, responsive auto-focus system in machine-readable symbol readers such as barcode readers, that eliminates mechanical elements.